A. Field of the Invention
This invention relates to mass spectrometry and in particular to improving signal-to-background ratio.
B. Background Art
The mass spectrometer is an extremely sensitive gas analyzer. A major problem in detecting extremely small concentrations of a sample substance is interference with the measurement due to small amounts of background substance remaining in the spectrometer from previous measurements. These background substances objectionably decrease the signal-to-background ratio of the detection signals.
It is well known in the art to use a vacuum pump to purge these background gases. However, the entire measurement apparatus of a mass spectrometer is in a vacuum chamber making a completely effective vacuum purge difficult. Additionally, some of the interfering substances have physically attached or chemically reacted with the walls of the vacuum chamber or elements within the vacuum chamber. During a subsequent analysis the substances desorbed from the walls and caused interfering background signals. The background signals interfered with detection and measurement of very small trace levels of components. This was a particular problem when the residual gas was the same substance as a substance being quantified in a subsequent measurement.
Another particularly difficult problem arose when analysis was performed to study trace nitrogen, trace water, or trace carbon dioxide because these substances were present at all times in the vacuum chamber. It was very difficult to completely remove these substances from the vacuum chamber and the substances thus tended to reside within the chamber.
One method of dealing with the problems of measuring trace levels of a substance in a mass spectrometer is taught in U.S. Pat. No. 3,974,380 issued to Rettinghaus. The system of Rettinghaus collimated the molecules of a sample gas into a beam rather than permitting the sample gas to fill up the vacuum chamber. The collimated beam of sample gas molecules was then ionized and analyzed. This collimated molecular beam approach decreased residual gas within the vacuum chamber by decreasing the amount of gas injected into the vacuum chamber and limited the surfaces which came in contact with the sample gas. The sample gas was formed into a collimated molecular beam using a nozzle as a gas inlet to form a jet of sample gas.
Condensing such a collimated molecular beam onto a cryogenic beam stop to minimize background contamination is shown in U.S. Pat. No. 4,039,828 issued to Pokar. The cryogenic beam stop caused the molecules of the beam to collect on the beam stop rather than on the walls of the vacuum chamber.
Pokar also taught eliminating a filament which was used to ionize gas molecules. This filament was a wire which was struck by the sample gas beam causing molecules of the sample gas to collect on the filament. The wire thereby provided a "memory" since collected sample gas molecules would remain on the filament until the next measurement. In Pokar this filament ionizer was replaced with a laser beam ionizer which ionized the sample molecules while providing no physical obstruction upon which the molecules could condense.
It is also known to pulse a fluid stream of sample into the vacuum chamber while using photoionization, such as a laser, in a region which is free of surfaces. The pulsed fluid stream system helped minimize the memory effect. This system is taught in U.S. Pat. No. 4,365,157 issued to Unsold.
It is also known that the signal-to-background ratio can be enhanced by increasing the amount of sample gas introduced into the mass spectrometer. For example, a closed ion source may be used to allow the ionizer to operate at a pressure of 10.sup.-3 Torr, nearly one hundred times greater than with a standard ion source. An orifice in the closed ion source allowed for a pressure differential of about 10.sup.3 Torr between the ion source and the mass filter of the spectrometer. However, in these methods residual gases remained within the mass spectrometer ion source itself and contributed interfering background signals during later analyses.
It is known that the signals resulting from the background gas molecules within a mass spectrometer can be distinguished from the signals of a sample gas in a molecular beam based upon their differential velocity distributions. See for example, Lyubimov et al, JETP Letters - SOV., 1968, page 49. It is further known that the useful sample gas signals and the interfering background signals can be separated electronically by means of a modulated molecular beam and phase sensitive detection.
The phase sensitive detectors thus may distinguish between these two types of signals and thereby distinguish between sample molecules and molecules of background substances. The use of phase sensitive detection to increase signal-to-background ratio is disclosed in U.S. Pat. Nos. 4,258,257 and 4,263,507 and Patent Cooperation Treaty Application No. PCT/SE81/00040 filed on Feb. 12, 1981 by Rosengren.